Objective Femtosecond laser direct writing has emerged as a powerful tool for fabricating three-dimensional photonic circuits inside transparent materials. However, conventional transverse writing geometry suffers from aberration-induced cross-sectional distortion, resulting in pronounced polarization sensitivity of the inscribed waveguides. Moreover, the single-focus point-by-point scanning strategy severely limits fabrication throughput when dealing with large-scale waveguide arrays or complex integrated structures. This work aims to develop a high-efficiency, polarization-insensitive waveguide fabrication strategy that simultaneously addresses the symmetry issue of transverse writing and the throughput bottleneck of serial scanning.

Methods A longitudinal femtosecond-laser multi-focus writing (LFLMW) technique based on a reflective phase-only spatial light modulator (SLM) was proposed. The SLM was loaded with computer-generated holograms designed by the GSW algorithm to convert a single incident beam into programmable multi-focus arrays at the focal plane of the objective lens. The sample was translated parallel to the laser propagation direction, ensuring that the axial direction of the multi-focus array coincided with the sample scanning direction. An optical system incorporating a 1030 nm, 300 fs, 76 kHz femtosecond laser amplifier, a 4f imaging system, and a 20× objective (NA = 0.4) was employed. Fused silica substrates with dimensions of 10 mm × 10 mm × 2 mm were used. Through systematic optimization of pulse energy, scanning speed, and inter-focus spacing, Type I waveguide arrays with different channel counts and cladding waveguides with varied track numbers were fabricated in a single longitudinal scan. Free-space end-face coupling at 633 nm was used to characterize the insertion loss, near-field mode profiles, and polarization dependence of the fabricated waveguides. Based on this technique, a 1×9 beam splitter was fabricated by dynamically switching between a ring-shaped hologram for the input cladding waveguide and a rectangular hologram for the output Type I waveguide array.

Results and Discussions Multi-focus arrays including 2×2, 3×3, 4×4 rectangular matrices and ring-shaped patterns with 8 to 16 foci were generated with high fidelity, as verified by CCD imaging at the Fourier plane. For Type I waveguide arrays, a 2×2 array was fabricated with a total pulse energy of 8.4 μJ and a scanning speed of 2 mm/s. The inter-focus spacing of 9 μm effectively suppressed thermal cross-talk between adjacent channels. All waveguides exhibited approximately circular cross-sections with an average mode-field diameter of ~5 μm. Average propagation losses of 1.1 dB/cm (TE) and 1.0 dB/cm (TM) were achieved. For 3×3 and 4×4 arrays, the total pulse energies were increased to 17.9 μJ and 21.7 μJ, respectively, with a reduced scanning speed of 0.4 mm/s to guarantee sufficient energy deposition at each focal spot. All channels maintained regular matrix arrangement without observable inter-channel distortion. Single-mode guidance was confirmed for both polarizations. The 3×3 array exhibited average losses of 0.4 dB/cm (TE) and 0.5 dB/cm (TM), while the 4×4 array achieved losses as low as 0.1 dB/cm (TE) and 0.2 dB/cm (TM).

Cladding waveguides with 8, 10, 12, 14, and 16 damage tracks were fabricated using a total pulse energy of 21.7 μJ and a scanning speed of 0.4 mm/s. The tighter focal spacing in the ring-shaped arrays induced significantly enhanced cumulative energy deposition through superimposed stress and thermal effects, pushing the local modification beyond the positive refractive index change regime into the negative change region dominated by micro-damage and material rarefaction. All cladding waveguides had a diameter of approximately 32 μm. Single-mode transmission was observed under both TE and TM polarizations for all track numbers. The measured propagation losses ranged from 3.62 to 4.83 dB/cm for TE polarization and from 2.98 to 4.17 dB/cm for TM polarization. Increasing the track number densified the cladding boundary, theoretically enhancing transverse confinement; however, it also introduced additional scattering interfaces, resulting in no monotonic trend in propagation loss as a function of track count.

A 1×9 beam splitter was demonstrated by cascading a 16-track cladding waveguide (core diameter 35 μm) as the input stage with a 3×3 Type I waveguide array as the output splitting stage. Hologram switching was executed without sample repositioning, ensuring precise alignment between the two sections. At 633 nm, total insertion losses of 2.20 dB (TE) and 1.37 dB (TM) were measured.

Coclusions A SLM-based longitudinal femtosecond-laser multi-focus writing technique was successfully developed for parallel and efficient fabrication of optical waveguides and integrated devices in glass. By active wavefront modulation, programmable multi-focus arrays were generated at the objective focal plane, enabling simultaneous inscription of multiple waveguide tracks in a single longitudinal scan. Type I waveguide arrays scaled from 2×2 to 4×4 and cladding waveguides with tunable track numbers from 8 to 16 were fabricated. Type I waveguide exhibited high cross-sectional circularity, polarization-insensitive guidance, and propagation losses comparable to those fabricated by conventional single-focus serial writing. Cladding waveguides shown a slightly larger TE/TM loss difference, while all cladding waveguides maintain stable single-mode guidance under arbitrary polarization. The cascaded integration of a cladding waveguide input and a Type I waveguide array output was achieved through dynamic hologram switching, realizing a 1×9 polarization-independent beam splitter with low insertion loss and high splitting uniformity at 633 nm. Compared with conventional serial writing, the proposed method theoretically reduces the processing time of an N-track structure to 1/N. Compared with transverse writing approaches, the longitudinal geometry eliminates aberration-induced cross-sectional asymmetry. This technique provides a promising route toward rapid prototyping and scalable manufacturing of high-performance photonic integrated devices for applications in quantum photonics, optical interconnects, and waveguide lasers.